serial: mxs-auart: add missed iounmap() in probe failure and remove

This driver calls ioremap() in probe, but it misses calling iounmap() in
probe's error handler and remove.
Add the missed calls to fix it.

Fixes: 47d37d6f

 ("serial: Add auart driver for i.MX23/28")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200709135608.68290-1-hslester96@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>